1. Field of the Invention
The present invention relates to photolithographic processes used in semiconductor integrated circuit device manufacturing and, in particular, to an infrared radiation post-exposure bake process of a chemically amplified resist layer for improving critical dimension control in a resultant patterned resist layer.
2. Description of the Related Art
Photolithographic processes are used in semiconductor integrated circuit device fabrication to define the dimensions and pattern of the circuit elements. FIGS. 1A-1C depict a series of typical photolithographic processes. In the processes, a resist layer 10 is first coated on a substrate (or other underlying layer) 12. Selected regions 14 of the resist layer 10 are then exposed to radiation (e.g. light emitted from a mercury arc lamp or excimer laser) through a mask 16, thereby changing the chemical structure and dissolution rate of these regions. Selective dissolution (i.e. development) of the resist layer produces a patterned resist layer 18.
Since the dimensional resolution that can be obtained in the patterned resist layer 18 (i.e. the smallest sized pattern that can be achieved) is proportional to the wavelength of the radiation used to expose the resist layer, deep ultraviolet (DUV) light (248 nm in wavelength) is often employed during fabrication of patterned resist layers with critical dimensions of 0.35 .mu.m and below. Commercially available DUV light sources can, however, deliver only a relatively low flux of DUV radiation to the resist layer. This limitation necessitates the use of a resist that is highly sensitive to DUV radiation.
"Chemically amplified" resists constitute one category of highly sensitive resists. Exposure of chemically amplified resists to DUV radiation generates a catalyst (typically an acid) therein. The catalyst then initiates a series of chemical transformations (reactions) in the resist layer which changes the structure and dissolution rate of the resist layer during development. Prior to development, this catalyst-initiated chemical transformation requires a thermal activation step, commonly referred to as a "post-exposure bake" (PEB). See H. Ito, Chemical Amplification Resists: History and Development within IBM, IBM J. Res. Develop., Vol. 41, No. 1/2, January/March, pp. 69-80 (1997); A. A. Lamola et al., Chemically Amplified Resists, Solid State Technology, August, pp. 53-60 (1991); and S. A. MacDonald et al., Chemical Amplification in High-Resolution Imaging Systems, Accounts of Chemical Research, Vol. 27, No. 6, pp. 151-158 (1994) for a detailed discussion of chemically amplified resist photolithographic processing, each of which is hereby incorporated by reference.
FIG. 2 illustrates the chemical reactions that occur in a conventional chemically amplified resist layer during processing. The process depicted in FIG. 2 employs a chemically amplified resist that includes: (i) poly(4-hydroxystyrene) (PHOST) that has been partially protected by t-butoxycarbonyl (t-BOC) to form poly(t-butoxycarbonyloxystyrene) resist polymers and (ii) a photochemical acid generator (PAG). The ratio of the phenyl-OH functional group to the t-BOC functional group (i.e. the ratio of m to n in FIG. 2) in the protected PHOST molecule is about 7:3. This resist composition is representative of several commercially available chemically amplified resists.
As illustrated in FIG. 2, when the PAG (for example, an onium salt PAG, such as triphylsulfonium hexafluorantimonate) is exposed to DUV radiation (h.nu.), an acid catalyst (depicted as H.sup.+) is generated in the DUV-exposed region. Upon heating (.DELTA.) of the chemically amplified resist layer during PEB, the acid catalyst initiates removal and subsequent decomposition of the acid labile t-BOC functional group and the release of CO.sub.2 and isobutylene. The acid catalyzed removal of the protecting t-BOC functional group produces a resist polymer product in the DUV-exposed region with enhanced polarity and, therefore, an increased aqueous base dissolution rate.
A PEB process serves two primary functions. First it provides energy for the resist polymer to overcome the chemical transformation activation energy barrier. Second, it increases the diffusion rate of the acid catalyst so that the catalyst can migrate short distances and catalyze a series of resist polymer decomposition sites.
Conventional PEB process steps are conducted in an oven at around 130.degree. C.-140.degree. C., for a period of about 90 seconds. These processes employ electrical resistance heaters, known as hot plates, that deliver heat evenly to the DUV-exposed and DUV-unexposed regions of the resist layer. As a consequence, the temperature and, therefore, the catalyst diffusion rate remain essentially uniform between the DUV-exposed and DUV-unexposed regions. This uniformity in catalyst diffusion rate enables the catalyst to freely migrate into, and to initiate undesired catalyzed chemical transformations in, the DUV-unexposed regions. During development, these catalyzed chemical transformations cause unintended changes in critical dimensions of the patterned resist layer. Such catalyst diffusion-induced critical dimension changes are difficult to characterize and predict since they are dependent on the geometry of the DUV-exposed region, as well as neighboring DUV-exposed and DUV-unexposed regions. This complexity limits the ability to compensate for catalyst diffusion-induced critical dimension changes by incorporating "corrections" into the mask.
Still needed in the art is a photolithographic process employing chemically amplified resists that minimizes catalyst diffusion-induced critical dimension changes and provides for improved control of patterned resist critical dimensions.